Linear hydraulic drive systems supply three orthogonal motions in programmable manipulators such as that described in U.S. Pat. No. 4,001,556 to Folchi et al. Such drive systems permit the gripper assembly to be positioned at any desired location within a prescribed area. Generally, a joint interconnects the X Y and Z linear arms which are driven relative to each other by motors by individual X Y and Z motors mounted at each interconnecting joint. In such X Y Z orthogonal manipulator systems, the gravity forces on the robot joints associated with the Z-axis is continuously present while the system is in operation and the arm drive motors turned on. Also, when the Z-axis motor is in the off condition, the gravity forces tend to collapse the arms in a downward direction.
Prior art systems for providing counterbalance forces to manipulators includes the use of counterweights such as in U.S. Pat. No. 3,661,276 to Wiesener. However, the use of weights and countermasses are disadvantageous in that they introduce a substantial additional mass into the system. The counterweights are often not only cumbersome, but require additional driving energy to accelerate the mass. The counterweights also take up additional space as well as often requiring substantial system design changes to accommodate load modifications. Another form of counterbalance measure used in manipulator type systems is that of a spring counterbalance such as that used in the material handling apparatus disclosed by Mosher in U.S. Pat. No. 3,608,743. The use of springs is disadvantageous in applications where a constant counterbalance force is required since the spring does not provide a constant force. One other form of means for counteracting the collapse of the arm system of a manipulator is the use of locking means such as brakes or air driven clamps for holding the arm in position when the motor driving power is removed. Here, however, no balancing force is provided when the arm drive motors are in operation, and the time lag inherent in brake systems for turn-on must be compensated or dealt with in the system operation.
In manipulator systems the weight of various tools, such as power screwdrivers, riveters, soldering irons, welding torches, etc., place differing substantial loads on the manipulator fingers and arms. These loads can be substantial and introduce errors into the motor drive servo system. For example, where a force compensation for the unloaded arms is set for 60 pounds and the program introduces a 30-pound tool to be picked up by the fingers to perform an assembly function, the servo offset null could be of sufficient magnitude to substantially effect the stability of the mechanical drive system. Thus, some automatic compensating force would be desirable to accommodate the various tool weights that would be grasped by the fingers.
It is an object of the present invention to provide a counterbalance system for a robot manipulator which provides a constant counterbalance force against the gravity forces. It is another object to provide a counterbalance system which provides a constant counterbalancing force in the Z-gravity direction for an X Y Z orthogonal manipulator. It is another object to provide a counterbalance system for a robot manipulator which is simple and adds minimal mass to the manipulator. It is a further object to provide a counterbalance system for a robot manipulator which automatically compensates for the various tool weights and objects handled by the robot grippers.